Looks like a typical AMO outtage to me. There are often gaps in AMO data collection, caused by routine server maintenance or software upgrades. If it goes on for more than a week then it's time to file a bug on bugzilla or contact Jorge.

Also note that the data gap happens right during the Firefox 35 release which began on the 13th. It's possible that Mozilla turned off data reporting during the update to avoid the usual outtages at AMO during updates. These outtages sometimes cause users to not receive critical extensions updates they need for compatibility with the new Firefox version.